In order to ensure good print quality in direct to paper (DTP) ink jet printing systems, it is desirable to hold the print media extremely flat in the print zone. Conventional approaches use electrostatic tacking of media to a moving transport belt that is held flat against a platen in the imaging zones. Conventional electrostatic tacking methods create a tacking field by primarily applying charges to the media side that is not in contact with the tacking surface (transport belt). The charges can be applied by well-known methods in the art including the use of various non-contact corona charging devices or the use of various pressured devices such as a biased roller. Generally, pressured devices such as biased roller charging can be preferred because the presence of mechanical pressure helps to tack stressful media such as curled or cockled media. In any case of conventional tacking, charge decay from the top of the media toward the tacking surface during the dwell times between imaging stations adversely affects the fields between the media and the imaging heads at certain stress media conductivity conditions where the charge decay rate is comparable to the dwell times. Moreover, in conventional tacking using a pressured device such as (bias transfer roll) BTR roll tacking, air breakdown charge exchange can occur between the media and the transport belt at the BTR exit when the media has lead edge curl away from the belt transport, and this greatly reduces tacking force on the lead edge of such curled print media, thereby causing undesirable low tacking force between the lead edge of the media and the belt transport.
For ease of discussion, we will discuss conventional charging using a BTR, but the general points made apply to all other forms of conventional charging (for example charging by other pressured bias charging devices or charging via non-contact corona devices). Conventional BTR charging applies initial charge primarily to the surface of the media that is facing the BTR rather than to the surface that is facing the transport belt, causing the charge to conductively migrate or “relax” toward the interface between the media and the belt transport during the dwell times between print zones. The time for this charge relaxation can vary by more than 6 orders of magnitude for media conditioned over extremes of relative humidity. This charge relaxation creates fields between the media and subsequent print heads past the 1st print head when the charge relaxation rate is comparable to the dwell time between printing head stations.
Another solution to avoiding fields between the media and print heads and the effect of media conductivity on these fields mentioned above involves the use of slots in the metal support below each imaging head. With appropriate optimized media charge conditioning past the BTR zone and slots that are sufficiently wide, that the fields between the media and the imaging heads can be kept very low below all of the imaging heads independent of media conductivity. However, very wide slots are not desirable for optimized maintenance of belt flatness in the imaging zones, and so some compromise in the slot width is typically needed. At a compromised narrower slot width, dependence on the media conductivity of the fields between the media and the heads can occur and this can cause similar issues mentioned for the non-slotted metal support.
Another disadvantage of conventional BTR charging methods occurs in media that has lead edge curl toward the BTR. Charge transfer from the BTR to the media is typically dominated by air breakdown, which includes charge transfer just past the BTR nip. With media curl toward the BTR, air breakdown past the nip can occur above and below the media, and this lowers the net charge on the lead edge and thereby greatly lowers the electrostatic tack force between the lead edge and the transport. This in turn greatly increases the danger of up-curl media damaging the downstream print heads. A conventional countermeasure to mitigate this phenomenon is to provide a pre-curl device prior to the BTR zone to ensure that the media lead edge is curled toward the transport. However, high curl toward the transport is not desirable and it is difficult to ensure that the media lead edge will always be curled down for all media and all environmental conditions if the pre-curl stage is confined to minimize the amount of curl.
There is a need in the art for systems and methods that facilitate providing a tacking system that allowed high tack force on the lead edge so that some level of up curl could be allowed while overcoming the aforementioned deficiencies.